[anth-volk]

Fixes #4147

Summary

• PolicyEngineCountry.calculate formats each value of a household calculation without axes by the variable's value_type. Since Make the SPM country tests unit tests, with two integration tests against the installed US model #4136 moved the SPM country tests onto stand-in simulations, every unit test that calls it uses float or int variables, so no test reached the Enum branch (result.decode()[entity_index].name) or the str branch (str(result[entity_index])). codecov/project fell 0.03 points on Make the SPM country tests unit tests, with two integration tests against the installed US model #4136.
• TestHouseholdCalculation gains test__calculate_returns_enum_names_and_plain_strings_without_axes, parametrized over an enum variable and a string variable. A stand-in simulation returns an EnumArray of the module's ExampleEnum or a NumPy string array, as the neighbouring tests stand in for _create_simulation. The test checks that the household cell holds the enum member's name ("SECOND") or the string, as a plain str.
• The axes path was already covered by the _serialize_axis_result tests.
• No production code changes.

Testing

• The two new cases, each run by node ID: both pass. A coverage run of just those two shows calculate's Enum and str branches executed.
• ruff format --check and ruff check on the changed test module, mypy (its configured files), and .github/scripts/check_changelog_fragment.sh against master: all pass.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.11%. Comparing base (22c4f36) to head (424cf47).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4148       +/-   ##
===========================================
+ Coverage   50.63%   88.11%   +37.47%     
===========================================
  Files         258      285       +27     
  Lines       14955    16577     +1622     
  Branches     2399     2712      +313     
===========================================
+ Hits         7573    14607     +7034     
+ Misses       6751     1292     -5459     
- Partials      631      678       +47     

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